1. Field of the Invention
The present invention relates generally to an apparatus for collection and treatment of engine fuel vapor in a fuel tank without emitting the vapor into the atmosphere. More particularly, it pertains to a fuel vapor treating apparatus provided with a canister, which collects fuel vapor, and a purging device for delivering the collected fuel to the intake passage of the engine.
2. Description of the Related Art
Fuel vapor treating apparatuses that are mounted on vehicles are known in the prior art. These apparatuses treat fuel vapor evaporated in fuel tanks without emitting the vapor into the atmosphere. Typically, such an apparatus is provided with a canister, which collects the fuel vapor from the fuel tank through a vapor line. The canister is filled with an adsorbent comprised of activated carbon or the like and has an opening connecting the inside of the canister with the atmosphere. A purge line, extending from the canister, is connected to an intake passage of an engine. The canister first adsorbs and collects the fuel vapor introduced by the vapor line with the adsorbent. Afterwards, the canister discharges only the residual gas, from which fuel components have been extracted, to the atmosphere. During operation of the engine, the canister purges the fuel adsorbed in the adsorbent into the intake passage through the purge line when necessary.
Japanese Unexamined Patent Publication 6-74107 discloses a fuel vapor treating apparatus. As shown in FIG. 6, the apparatus has a canister 104 provided with a partition 101, which defines a main chamber 102 and a sub-chamber 103. An adsorbent 105 occupies the chambers 102, 103. A hole 106 provided in the partition 101 connects the two chambers 102, 103 to each other and serves to regulate the flow of fluid therebetween. The main chamber 102 is connected to a fuel tank 108 by a vapor line 107. The sub-chamber 103 communicates with the atmosphere through a discharge opening 109. The main chamber 102 is connected to an intake passage 112 of the engine 111 by a purge line 110. The fuel vapor conveyed into the main chamber 102 by the vapor line 107 is first adsorbed by the adsorbent 105 and collected therein. When the concentration of the fuel components adsorbed in the adsorbent 105 becomes high in the main chamber 102, the fuel vapor is diffused in the sub-chamber 103 through the hole 106. The hole 106 which regulates the flow of fuel vapor from the main chamber 102 to the sub-chamber 103 results in less adsorption of fuel in the sub-chamber 103 than in the main chamber 102. Accordingly, only the residual gas from which fuel components have been extracted is discharged into the atmosphere from the sub-chamber 103 through the opening 109. During operation of the engine, the fuel adsorbed in the two chambers 102, 103 is purged into the intake passage 112 through the purge line 110 when necessary.
In recent years, fuel vapor escaping into the atmosphere from fuel tank filler openings during refueling has been recognized as a source of pollution. Therefore, it is necessary to appropriately treat the fuel vapor, especially since a large amount of fuel vapor escapes into the atmosphere during refueling.
U.S. Pat. No. 4,714,172 discloses an apparatus which treats fuel vapor emitted during refueling. This apparatus is provided with, in addition to the vapor line, a special breather line connected between the fuel tank and the canister. The breather line has a diameter that is larger than the diameter of the vapor line. A fuel nozzle is inserted into a filler hole of a filler pipe provided on the fuel tank. A seal provided in the filler pipe seals between the periphery of the nozzle and the filler pipe when the nozzle is inserted. Accordingly, the large amount of fuel vapor emitted from the fuel tank during refueling is conveyed to the canister through the breather line. The fuel components of the fuel vapor conveyed to the canister is collected in the adsorbent. The residual gas from which fuel components have been extracted is discharged into the atmosphere by a control valve provided in the discharge opening. The breather line is required to be opened during refueling and closed at other times. A differential pressure regulating valve provided in the breather line is opened according to a pressure increase caused by the evaporation of fuel vapor during refueling. Furthermore, the periphery of the fuel nozzle, sealed by the seal, prevents the fuel vapor from escaping into the atmosphere.
Hereafter, the process where fuel vapor evaporated during refueling is conveyed to the canister for treatment is referred to as "Onboard Refueling Vapor Recovery (ORVR) treatment" while treatment of the fuel vapor at other times is referred to as "normal treatment."
During ORVR treatment, the typical amount of fuel vapor conveyed to the canister through the breather line is approximately forty-five liters per minute. This amount is very large when compared to that during normal treatment, which is approximately one liter per minute. Therefore, a desirable canister employed for ORVR treatment is one having a small ventilation resistance. However, when a canister similar to the canister 104 disclosed in the Japanese Unexamined Patent Publication 6-74107 is employed, there is a problem with its ventilation resistance. The ventilation resistance within the canister is relatively large since the canister has a hole that has a regulating effect between the two chambers. However, a large amount of fuel vapor is conveyed to the canister during ORVR treatment. Therefore, the fuel vapor does not flow smoothly to the canister from the fuel tank. This raises the internal pressure of the fuel tank and decreases the refueling efficiency.
To prevent such a decrease in refueling efficiency, the partition may be eliminated from the canister. However, this will also eliminate the regulating effect of the hole. Without the regulating effect, the fuel vapor drawn into the canister is diffused throughout the entire adsorbent. This causes the fuel concentration to become uniform throughout the adsorbent. Accordingly, the more uniform the fuel concentration of the entire adsorbent becomes, the more difficult it is to maintain low fuel concentration adsorbent in the vicinity of the discharge opening. A high concentration of fuel in the vicinity of the opening increases the possibility that the fuel components will escape into the atmosphere through the discharge opening. Thus for ORVR treatment, it is desirable to suppress uniform diffusion of fuel vapor while maintaining a low ventilating resistance inside the canister.